Session 5 Regulation of Ca

• How is calcium signaling initiated?

  • GPCR
  • ion channels
• Role of Ca2+ in cellular events?

  • Bind and regulates proteins - alters their functions
  • Binds to trigger proteins regulates levels of Ca2+
• Serum Ca2+ levels?

  • 2.2 - 2.6 mmol/L
  • Free Ca2+ -> 1.3 - 1.5 mmol/L
• Dysregulation of Intracellular Ca homeostasis can lead to?

  • Metabolic diseases
  • Cardiovascular diseases
  • Neurological diseases
  • Cancer
• How is calcium homeostasis achieved?

  • Relative permeability of Ca by plasma membrane
  • Ability to expel by plasma membrane (PMCA, NCX)
  • Ca buffers
  • Intracellular calcium stores (S/ER, mitochondria)
• What is PMCA?

  Plasma membrane Ca ATPase
• PMCA activity is directly influenced by [Ca2+]i?

  • [Ca2+]i increases
  • Ca2+binds to calmodulin (Ca2+-binding protein– a trigger protein)
  • Ca2+-calmodulin complex binds and activates Ca2+-ATPase
  • Ca2+-ATPase removes Ca2.
• In NCX (sodium calcium exchanger), during resting stage, 3 Na goes in and 1 Ca goes out.
• In NCX (sodium calcium exchanger), during depolarized stage, 3 Na goes out and 1 Ca goes in.
• Ca2+ Buffers limit Ca2+ diffusion (depends on binding molecules and level of saturation)
• Excess of intracellular Ca2+ is taken up by mitochondrial calcium uniporter (MCU) in mitochondria.
• Excessive mitochondrial Ca2+ uptake leads to loss of Ca2+ via permeability transition pore (mPTP), initiating cell death – apoptosis.
• How is the [Ca2+]i Elevated and Returned to Basal Levels?

  • Ca2+ influx via plasma membrane (VOCC/ROCC)
  • Ca2+ release from the rapidly releasable intracellular stores (IP3 , Ryr receptors)
  • Ca2+ release from the non-rapidly releasable intracellular stores (Mitochondrial NCX)
• Voltage-operated/gated Ca2+ channels (VOCC/VGCC)?

  • found in neurons, endocrine cells and myocytes
  • highly selective (L-type - cardiac and smooth muscle; T-type - neurotransmitter)
  • Targets Ca channel blockers (used to treat hypertension, angina and arrhythmias)
• Receptor-Operated Ca2+ Channels (ROCC)?

  • non-selective
  • E.g., ionotropic glutamate receptors (NMDA type), nicotinic acetylcholine receptors
  • targeted to treat in pain killers and as anaesthesia
• Role of Ca2+ in Contraction?

  • Calcium binds to its binding sites on troponin C
  • Tropomyosin moves away from the myosin-binding sites on actin thus exposing it
  • Myosin heads bind to actin, forming cross bridges between the thick and thin filaments
  • The interaction between actin and myosin leads to the sliding mechanism during which sarcomeres shorten leading to the muscle contraction.
  • ATP is required
• Ryanodine Receptors (RyR) - Calcium - induced Ca2+release (CICR)
• IP3 receptors are ligand- gated channels that are activated by the second messenger: inositol trisphosphate (IP3).

  IP3 is produced by the action of ligands on the GPCRs

  When IP3 binds to these receptors, it induces a conformational change that leads to the opening of channels, resulting in Ca2+ efflux.
• In skeletal and cardiac muscle, calcium binds to troponin C which exposes binding sites on actin.

  In smooth muscle, calcium binds to calmodulin, which leads to activation of myosin light chain kinase (MLCK) • MLCK phosphorylates myosin which then binds to actin.
• In skeletal muscles/cardiac - VGCC and RyR

  In smooth muscles - IP3
• How are stores refilled?

  1. Recycling of released (cytosolic) Ca2+ e.g., cardiac myocyte
  2. VOCC and/or capacitive or store-operated Ca2+ entry (SOCE) (in non-excitable cells, but also in excitable cells).
• Capacitive Ca2+ Entry?

  • Influx of Ca2+ via store-operated channel (SOC) to replenish intracellular stores (mainly ER).
  • STIM 1(Stromal interaction molecule 1) – S/ER membrane located Ca2+ sensor; stimulated upon calcium depletion in ER; it interacts with Orai
  • Orai 1 (Calcium release-activated calcium channel protein 1) located in plasma membrane; upon activation it allows for calcium influx.
  • Once ER stores are refilled, STIM 1 dissociates from Orai channels.